Message transfer agents are programs responsible for delivering electronic mail messages. Upon receiving a message, a message transfer agent stores the message locally, analyzes its recipients, and either delivers the message locally or routes it to another message transfer agent. The prevalence of undesired electronic mail messages or spam has hindered the entire system of electronic messaging. Many users find that they receive so much undesired electronic mail that it is difficult to sort through and find the desired mail. Systems are available that check for certain key words and quarantine messages with those key words. Despite the existing techniques for controlling spam, however, spam continues to be a major problem in electronic mail systems.